Laminates of metals with polymers are known. Examples include pipes coated with a polymer to decrease corrosion while maintaining the mechanical properties of the metal, food packaging that combines the shelf life of the metal with the sealing properties of the polymer, and fasteners such as coated nails or screws with improved corrosion resistance. Adhesion between the polymer and the metal is often an issue and there have been several attempts to improve interlayer adhesion. For example, U.S. Pat. No. 7,267,883 prepares a laminate of a metal with a polyimide film. To improve adhesion, they use a specific polyimide formulation, plasma treat the polyimide, and then use chemical vapor deposition of the metal. U.S. Pat. No. 4,888,237 discloses flame treatment of polypropylene prior to metallization to improve the interlayer adhesion.
U.S. Pat. No. 6,723,431 teaches multilayer films, including one made from polypropylene blended with maleic anhydride-grafted polypropylene. The polypropylene surface is corona-treated and then extrusion coated with ethylene-vinyl alcohol (EVOH) copolymer. The EVOH skin comprises the surface for aluminum deposition. This is an intricate, multi-step process.
Blends of polyolefin with a grafted polyolefin are known. U.S. Pat. No. 4,087,587 teaches blends of polyethylene with a grafted polyolefin. The grafted polyolefin is polyethylene that has been reacted with an unsaturated fused ring carboxylic acid anhydride. The blends adhere well to a variety of substrates, including metals. Another blend of polyolefin with grafted polyolefin is disclosed in U.S. Pat. No. 4,416,944. The blend is used to make polymer-metal composites such as aluminum/polymer blend/copper “sandwiches.” For other examples of grafted polyolefin blends, see U.S. Pat. Nos. 3,856,889, 4,460,745, 4,452,942, and 5,367,022. While these blends give improved adhesion versus polyolefin without grafted polyolefin, the adhesion is not sufficient for some applications.
Polymer surfaces are sometimes treated with plasma to promote adhesion. For example, Annual Technical Conference-Society of Plastics Engineers (2006), 64th at page 418 discloses that plasma treatment of polypropylene improves its adhesion to UV-curable inks. A presentation by Enercon Industries Inc. at TAPPI 9th European Place Conference 2003—Rome, Italy entitled “Atmospheric Plasma—The New Functional Treatment for Extrusion Coating and Lamination Processes” discloses that LDPE treated with plasma exhibits improved adhesion with acetate-film acrylic-adhesive tape. In another example, U.S. Pat. No. 4,897,305 discloses plasma treatment of polypropylene with aliphatic hydrocarbon vapors to improve its adhesion to metals.
Plasma treatment of metal surfaces to improve adhesion to polymers appears to be known but is rarely reported. U.S. Pat. No. 4,366,184, for instance, teaches to improve adhesion of a silicone elastomer to a metal substrate by plasma treating the metal substrate in the presence of an organosilicon compound. In another example, published Japanese application 11-106947 teaches treatment of metal surfaces with low-temperature plasma to impart peel-resistance to an overcoating on the metal.
Despite earlier progress, the need remains for metal-polymer laminates with high interlayer adhesion. Poor interlayer adhesion renders a laminate unsuitable for many applications. A simple way to make laminates with high interlayer adhesion would be valuable.